1. Field of the Invention
This invention relates to an optical information reproducing apparatus and, more particularly, to an optical apparatus capable of exact track access in relation to an optical information reproducing operation.
2. Description of the Related Art
In general, an optical information reproducing apparatus uses an optical pick-up having an objective lens to retrieve information recorded on a circular recording medium known as an optical disc. A plurality of sectors are defined radially on the disc, and a plurality of tracks are formed concentrically thereon. Information is recorded in the form of a pit and a number of pits are formed on each track, the length of each pit differs according to what information is recorded. The operation of reading out or reproducing information recorded on the disc is effected as follows:
First, a laser beam is applied to a preset track position, via the pick-up, and is reflected toward the objective lens. The amount of reflected laser beam varies depending on the presence or absence of a pit. When the laser beam is applied to a position having no pit, an intense reflected light is incident on the objective lens. In contrast, when the laser beam is applied to a pit, the beam diffracts, with the result that part of the reflected light is not transmitted to the objective lens, thus reducing the amount of reflected light incident on the objective lens. Therefore, whether information is on the track is detected from the amount of reflected light.
When information is reproduced by applying a laser beam to the optical disc, it is necessary to control the movement of the optical pick-up or objective lens so that the laser beam can always be applied to a preset pit position in a preset track. Accordingly, when the laser beam spot is caused to deviate from the preset pit position, because of eccentricity of the disc or distortion of the track, a servo system is operated to move the optical pick-up to a position where the pit comes directly under the lens. The above control operation is generally called tracking servo control. Known tracking servo control methods include, the three spots method, the wobbling method, the push-pull method, among others.
Japanese patent publication No. 58-55566 discloses a tracking servo control in which an electrical feedback circuit is provided for controlling a motor device used for shifting a lens, the amount of the shift depending on a voltage supplied from a photoelectric converter which converts light reflected from a track into electrical signals.
During the accessing of a track, the optical pick-up up or objective lens is moved over the disc surface. To determine the position of the pick-up or objective lens after the movement has commenced, information relating to the reference position and the distance travelled by the optical pick-up or objective lens is required. To obtain the information on the distance travelled, the light reflected from the disc is transmitted to photosensors, via the optical pick-up including the objective lens. A tracking signal is derived from a difference between two outputs of the photosensors, and is converted into binary form. The binary coded output is supplied to a counter circuit which generates count data representing the number of tracks traversed by the objective lens or optical pick-up, which represents the distance travelled by the optical pick-up or objective lens. In this way, it is always possible to determine the distance travelled by the optical pick-up or objective lens.
However, the tracking signal inevitably picks up a D.C. component when the optical pick-up or objective lens traverses tracks on the disc. Further, when either or both of them are caused to vibrate by some external phenomena, it is likely to deviate from the center of the optical axis of a laser beam. In the event of this occurring, the tracking signal will have an asymmetrical waveform. The result is that binary coded values gained can be inaccurate because of the asymmetry of the tracking signal. Therefore, precise count data cannot be derived by the counter, making it impossible to precisely determine where the optical pick-up or objective lens is located after the movement thereof has begun.